1. Field of the Invention
The present invention relates to a liquid crystal display panel driving device and method for improving the display characteristics of a liquid crystal display panel.
The invention also relates to a response-speed improving circuit in a liquid crystal display panel.
2. Description of the Related Art
Some conventional devices for improving the display characteristics of a liquid crystal display panel aim at improvement in the response speed of a liquid crystal, such as ones described in Japanese Patent Laid-Open Application (Kokai) No. 3-96993 (1991) and in U.S. Pat. No. 5,119,084.
For example, Japanese Patent Laid-Open Application (Kokai) No. 3-96993 (1991) describes the following problems in conventional liquid crystal display devices. That is, xe2x80x9cit is known that in liquid crystal display devices, a change in the orientation of liquid-crystal molecules for a change in the electric field delays due to the viscosity of the liquid crytal used in the device, resulting in large rise time and fall time, i.e., inferior transient response characteristics. When displaying a still image by a liquid crystal display device, such inferior transient response characteristics of liquid crystals cause no problem. However, when displaying a moving image by a liquid crystal display device, appearance of afterimage in a displayed image thereby to degrade the quality of the reproduced image is a problem.xe2x80x9d
In order to reduce the occurrence of such a problem, Japanese Patent Laid-Open Application (Kokai) No. 3-96993 (1991) discloses the following afterimage cancelling circuit for a liquid-crystal display device.
In the afterimage cancelling circuit disclosed in Japanese Patent Laid-Open Application (Kokai) No. 3-96993 (1991), a difference signal, representing the difference between image signals to be displayed on a liquid crystal display device and image signals separated by one-frame period or one-field period, is generated. When the value of the difference signal is larger than a predetermined value, the difference signal is added to an input image signal in order to prevent appearance of afterimage in an image displayed on the liquid crystal display device. When the value of the difference signal is smaller than the predetermined value, the difference signal is treated as noise, and an input image signal is output without adding the difference signal, otherwise the difference signal is subtracted from the input image signal in order to output an image signal having reduced noise.
The afterimage cancelling circuit disclosed in Japanese Patent Laid-Open Application (Kokai) No. 3-96993 (1991) will now be described with reference to FIG. 9.
In the afterimage cancelling circuit shown in FIG. 9, an image signal input to an input terminal 901 is supplied to subtracters 902 and 904 as a signal to be subjected to subtraction, as well as to an adder 907.
An output signal from a coefficient circuit 905 is supplied to the subtracter 902 as a subtraction signal. An output signal from the subtracter 902 is stored in a memory 903. For example, the memory 903 comprises a FIFO (first-in first-out) memory, or two memories configured so as to alternately perform writing and reading for every one-field period (or one-frame period), so that an image signal which precedes the current image signal for a one-field (or a one-frame) by the one-field period (or one-frame period) is read from the memory 903 and is supplied to the subtracter 904 as a subtraction signal.
The subtracter 904 uses the current image signal supplied from the input terminal 901 as a signal to be subjected to subtraction, and supplies a difference signal obtained by subtracting an image signal which precedes the current image signal for a one-field period (or a one-frame period) by the one-field period (or the one-frame period) read from the memory 903 from the current image signal to coefficient circuits 905 and 906.
The coefficient circuit 905 multiplies the difference signal output from the subtracter 904, for example, by a coefficient smaller than 1, and supplies the subtracter 902 with the resultant signal as a subtraction signal.
That is, according to the operation of a loop of the subtracter 902xe2x86x92the memory 903xe2x86x92the subtracter 904xe2x86x92the coefficient circuit 905xe2x86x92the subtracter 902, a difference signal representing the difference between image signals separated by a one-frame period or a one-field period (a motion detection signal) is output from the subtracter 904.
The difference signal output from the subtracter 904 is supplied to the coefficient circuit 906.
When the value of the difference signal supplied from the subtracter 904 to the coefficient circuit 906 is within a range of 0xcx9c+a or 0xcx9cxe2x88x92a, i.e., smaller than a predetermined value |a|, an output signal obtained by multiplying the input signal by a coefficient having a polarity inverse to the polarity of the input signal is output to the adder 907, which outputs an image signal obtained by subtracting the difference signal from the input image signal to an output terminal 908. In the output signal in this state, noise is reduced in the image signal.
When the value of the difference signal supplied from the subtracter 904 to the coefficient circuit 906 is outside the range of 0xcx9c+a or 0xcx9cxe2x88x92a, i.e., larger than the predetermined value |a|, an output signal obtained by multiplying the input signal by a coefficient having the same polarity as the polarity of the input signal is output to the adder 907, which outputs an image signal obtained by adding the difference signal to the input image signal to the output terminal 908. The output signal in this state can cancel afterimage in an image displayed on the liquid crystal display device.
However, studies done by the inventor of the present invention have revealed that the afterimage cancelling circuit described above has problems. That is, for example, when applying a signal voltage of +5 V to an image signal for a frame and then applying a signal voltage of +15 V to an image signal for the next frame, the difference between the applied signal voltages is 10 V. On the other hand, when applying a signal voltage of +10 V to an image signal for a frame and then applying a signal voltage of +20 V for an image signal for the next frame, the difference between the applied signal voltages is also 10 V. However, when correction is performed by using the same difference signal for the two cases, optimum correction is not always obtained. This is because a liquid crystal behaves differently depending on the value of the level of the signal applied thereto. That is, even if the same difference signal is applied, optimum correction is not obtained unless the value of the level of the signal applied to the liquid crystal is considered. This fact is not taken into consideration in Japanese Patent Laid-Open Application (Kokai) No. 96993 (1991).
For example, in display devices using twisted nematic liquid crystals, vertically aligned liquid crystals and PDLC""s (polymer dispersed liquid crystals), the response speed greatly differs depending on the absolute values of the level of the signal for the preceding frame and the level of the signal for the current frame.
Although the response speed from 0% to 100% of the maximum luminance level is sufficiently high, the response speed from 0% to 10% of the maximum luminance level is very low.
As described above, since the response speed greatly differs depending on the values of the level of the signal for the preceding frame and the level of the signal for the current frame, an optimum response speed cannot be obtained according to the conventional correction method that does not depend on the signal level.
For example, if adjustment is performed so as to be adapted to a signal level having a high response speed, sufficient improvement is not obtained for a signal level having a low response speed. On the other hand, if adjustment is performed so as to be adapted to a signal level having a low response speed, an image having prominent noise is, in some cases, obtained because of excessive correction.
It is an object of the present invention to solve the above-described problems.
According to one aspect, the present invention which achieves the above-described object relates to a liquid crystal display panel driving device including first signal level detection means for detecting a signal level of an input image signal, memory means for delaying the input image signal by an arbitrary constant time period, second signal level detection means for detecting a level of a signal output from the memory means, and corrected-image-signal calculation means for correcting the input image signal based on an output from the first signal level detection means, an output from the second signal level detection means, and the arbitrary constant time period, and for outputting the resultant image signal.
According to another aspect, the present invention which achieves the above-described object relates to a liquid crystal display panel driving device including first encoding means for encoding a level of an input image signal, memory means for delaying the input image signal by an arbitrary constant time period, second encoding means for encoding a level of a signal output from the memory means, corrected-image-signal calculation means for obtaining a correction value based on outputs from the first and second encoding means, and addition-subtraction means for adding/subtracting the corrected value to/from the input image signal.
According to still another aspect, the present invention which achieves the aboved-described object relates to a liquid crystal display panel driving device including first encoding means for encoding a level of an input image signal, second encoding means for encoding the level of the input image signal, memory means for delaying an output from the second encoding means by an arbitrary constant time period, corrected-image-signal calculation means for obtaining a correction value based on an output from the first encoding means and an output from the memory means, and addition-subtraction means for adding/subtracting the correction value to/from the input image signal.
According to yet another aspect, the present invention which achieves the aboved-described object relates to a liquid crystal display panel driving device including encoding means for encoding a level of an input image signal, memory means for delaying an output from the encoding means by an arbitrary constant time period, corrected-image-signal calculation means for obtaining a correction value based on an output from the encoding means and an output from the memory means, and addition-subtraction means for adding/subtracting the correction value to/from the input image signal.
In one embodiment, the corrected-image-signal calculation means corrects the input image signal so as to maintain a linear relationship between the difference between the output from the first signal level detection means and the output from the second signal level detection means and a step level of a display luminance of a liquid crystal display panel, based, in depedence upon a value of a step response output in a relationship between a driving voltage and the display luminance of the liquid crystal display panel after the arbitrary constant time period, on levels before and after the step.
In another embodiment, the corrected-image-signal calculation means comprises a look-up table.
In still another embodiment, the arbitrary constant time period is a time period until the input image signal again drives the same pixel of the liquid crystal display panel.
According to yet a further aspect, the present invention which achieves the above-described object relates to a liquid crystal display panel driving method including the step of driving a liquid crystal display device by an image signal corrected so as to maintain a linear relationship between a step level of the image signal and a step level of a display luminance of the liquid crystal display panel, based, in depedence upon a value of a step response output in a relationship between a driving voltage and the display luminance of the liquid crystal display panel after an arbitrary constant time period, on levels before and after the step.
In one embodiment, when the level of the current image signal is low, finer correction is performed than when the level is high.
In another embodiment, the arbitrary constant time period is a time period until the input image signal again drives the same pixel of the liquid crystal panel.
According to the present invention, by correcting the driving signal for the liquid crystal display panel to an optimum value in accordance with the level of the current input image signal and the level of the past input image signal driving the same pixel, the display characteristics of the liquid crystal display panel can be improved.
It is thereby possible to display an image having a smaller afterimage even when displaying a moving image. Hence, it is possible to use even a liquid crystal display panel having a low response speed for use in displaying a moving image.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.